Process and apparatus for the utilization of low heat value fuels



Jan. 9, 1968 J. A. KIVLEN ET AL 3,362,902

PROCESS AND APPARATUS FOR THE UTILIZATION OF LOW HEAT VALUE FUELS 2Sheets-Sheet 1 Filed July 7, 1965 FIGURE I Catalytic Cracking Cu'mlysiRegeherutor 3 E R U m F INVENTORS JOHN A. KIVLEN JAMES F. HARRIS IRVINGD. CRANE BY 4 Mi.

PATEN T ATTOR NEY J. A, KIVLEN ET AL PROCESS AND APPARATUS FOR THEUTILIZATION Jan. 9, 1968 OF LOW HEAT VALUE FUELS 2 Sheets-Sheet 2 FiledJuly 7, 1965 26 cozwanEoo mm cozomm 8522;

cozuww cocu coo m INVENTORS JOHN A. KIVLEN JAMES E HARRIS IRVING D.CRANE BY WMMYQ.

PATENT ATTORNEY United States Patent ice 3,362,902 PROCESS AND APPARATUSFOR THE UTI- LIZATION 0F LSW HEAT VALUE FUELS John A. Kivlen and JamesF. Harris, Sparta, and Irving D. Crane, Morristown, N..I., assignors toEsso Research and Engineering Company, a corporation of'Delaware FiledJuly 7, 1965, Ser. No. 470,017 6 Claims. (Cl. 208113) This inventionrelates to a process and apparatus for the utilization of low heat valuefuels. More particularly, the invention relates to a process andapparatus for utilizing the heating value of a CO containing gas in a COfurnace.

Catalysts employed in the processing of hydrocarbon oils are regeneratedby burning off carbonaceous matter and the operation produces aregenerator flue gas. This gas contains from about to about volumepercent carbon monoxide; the remainder consisting of inert gasesincluding carbon dioxide, nitrogen and water vapor.

CO has a heating value of about 4347 B.t.u./lb., and

under some circumstances it is considered worthwhile to burn it torecover the heat of combustion. The principal problem lies in raisingthe temperature of the CO containing gas, sometimes hereinafter referredto as the low heat value fuel, to the temperature at which the CO willignite and burn completely. The ignition temperature of CO is about 1130F., and since regenerator off gas is sometimes available at relativelylow temperatures ranging from 400-1100 F. it is necessary to heat itwith a high heat value fuel to burn the CO. For purposes of thisdescription low heat value fuels are those with a heating value of lessthan 10,000 B.t.u./lb. and high heat value fuels are those with aheating value of more than 10,000 B.t.u./lb. The object of thisinvention is to provide a process and apparatus in which a low heatvalue fuel containing CO is heated and completely burned employing aminimum amount of expensive high heat value fuel. Another object of thisinvention is to use the products of CO combustion to heat petroleumprocess streams and to. make steam. A more specific object of thisinvention is to provide a CO furnace for use in conjunction with acatalytic cracking unit.

These objects and additional objects and advantages of the inventionwill be apparent from the description which follows.

We have found that by burning the CO in two distinct combustion stagesand by contacting the fluids to be heated with the'hot combustion gasesin a particular manner, an eificient CO utilization system is achieved.

The invention will be more fully described with reference to theattached drawings in which:

FIGURE 1 is a schematic view of the CO furnace showing its relationshipto the catalytic cracking unit and the regenerator.

FIGURE 2 is a side view of the CO furnace, particularly the combustor,the transition section and the heat transfer section.

FIGURE 3 is a cross-sectional view taken along line 3,3 of FIGURE 2.

Referring to FIGURE 1, reference numeral 1 denotes a catalystregenerator. Details of the regenerator and the regeneration procss willbe omitted since they are not pertinent to the description of theinvention. Regenerator off gas is carried in line 2 to Y 3 where the gasstream 3,362,902 Patented Jan. 9, 1968 is divided into two streams whichare usually of approximately equal volume. Regenerator off gas containsfrom about 5 to about 15 volume percent CO. Analysis of one regeneratoroff gas revealed the following composition:

Component Percent by Volume Percent by Weight The temperature of the gasin line 2 ranges from 400- 1100 F. depending on regenerator conditionsand heat losses. It the gas is passed through a conventional heatexchange facility such as a waste heat boiler after it leaves theregenerator, the gas temperature can drop to a range of 400 700 F.Conventional CO furnaces are not suited to these cold gas feeds becauseof the exorbitant amounts of high heat value supplementary fuel requiredto obtain complete combustion of the CO. The invention is not limited toregenerator off gases as low heat value fuel. Any waste gas containingfrom 5-15 volume percent CO or any other combustible low heat value fuelcan be burned in the process and apparatus described herein.

Approximately equal volumes of gas are passed by lines 4 and 5 to gasdistributors 6 and 7 respectively. High heat value fuel is fed by line 8to a burner shown generally by reference numeral 9. Primary air issupplied to the burner by line 10. Air for combustion of CO in the firststage is also supplied by line 10. Following two-stage combustion inhorizontal combustion zone 11, the com bustion products pass throughtransition section 12 into and through convection section 13. Spentgases are vented to the atmosphere through stack 14. In the convectionsection, steam is generated by passing feed water into the section byline 15 andrecovering steam through line 16. A hydrocarbon oil to becracked is preheated by passing it through line 17, crossover line 18and line 19, and then to catalytic cracking unit 20. Details of thecatalytic cracking unit will be omitted since the function of the unitis not a part of the invention.

A more detailed description of the CO furnace will be provided withreference to FIGURES 2 and 3. Like reference numbers in FIGURES 1, 2 and3 are used to designate like elements.

Referring to FIGURE 2, the combustion section or combustor of thefurnace shown generally by reference numeral 11 is disposedhorizontally. It is provided with an end wall 21 containing one or moreopenings 22 for one or more burners 9. The Wall or walls 23 of thecombustion section are circular or rectangular as desired. Thedownstream end of the combustor is unobstructed except for a downwardlyhanging wall or baflie wall 24. When the combustor is circular incross-section, the wall is a segment of a circle. When the combustor isrectangular in cross-section, the wall extends across the top. The walloccupies from 10-35 percent of the cross-sectional area at thedownstream end of the combustor. The function of the hanging or bafilewall will be discussed subsequently. The combustor as well as thetransition section and the convection section of the furnace comprise ametal shell lined with a suitable insulating refractory shown generallyby reference numeral 25.

As stated previously, the CO containing gas is preferably divided intotwo streams of approximately equal volume. However, from 30-70 volumepercent can be fed to the first stage if desired. The streams passthrough pipes 4 and 5 into distribution ducts 6 and 7 respectively. Inthe preferred embodiment, the combustor is circular in cross-section andthe distribution ducts are annular. Referring to FIGURE 3, which is across-sectional view taken along line 3-3 of FIGURE 2, the distributionduct comprises an outer shell 26, an air plenum 27, and a CO plenum 28.The external surface of the combustor 23 and the insulation layer 25 arepierced by a plurality of spaced inlets shown generally by referencenumeral 29. While FIGURE 3 shows eight inlets spaced 45 of arc apart,any other desired number of inlets can be used. Dampers 30 and 31 areset to cause the CO containing gas to flow into and around plenum 28 sothat the gas distribution between plenum 28 and the first stage plenumis approximately equal, i.e. the dampers in ducts 4 and 5 are used tocontrol gas fiow at Y 3 of FIGURE 1. The internal configuration ofdistribution ducts 6 and 7 is the same except that duct 6 does not havean air plenum. Air for the second stage of C combustion is supplied bypipe 32 to duct 7. While we have disclosed two stages of CO combustionand two distribution ducts, we wish to include the embodiment of threeor more stages of combustion and three or more distribution ducts.

Burner 9 can be any conventional type of burner designed for burninggas, oil or both types of high heat value fuel. The burner is operatedwith sufficient excess air to provide combustion air for the first stageof CO combustion. A range of 60l20 percent excess air is satisfactory.In addition, the burner is operated to provide a heat output suificientto heat the first stage CO combustion area to a temperature of 1l001400F., preferably 1150-4200 F. We have found that temperatures in thisrange are sufiicient to heat, ignite and completely burn the CO in a gascontaining from -15 percent CO entering the combustor at a temperatureranging from 400- 1100 F. The first CO distribution duct is located at apoint downstream of the burner at which combustion of the high heatvalue fuel is 40-100 percent complete. Of course, location of the ductsdepends on the cross-sectional area of the cornbustor, number and typeof burners, type of high heat value fuel, etc. Conventional heatengineering techniques and calculations can be used to locate the COducts. Generally speaking, the minimum distance between the burner endof the combustor and the inlet to the first CO burning stage will be .02/A to /A where A is the cross-sectional area of the combustor.

As indicated above, about one-half of the cold low heat value fuel, i.e.the CO containing waste gas, is mixed with air and the hot combustiongas from combustion of the high heat value fuel, i.e. the supplementaryfuel and the CO in the mixture is burned in the first stage. The hotcombustion gases from the first stage pass to the sec- 0nd stage. In thesecond stage, the other portion of the relatively cold CO containing gasis introduced into the combustion zone along with combustion air and thesecond stage of combustion takes place. The second stage of combustiondiffers from the first stage in that the high temperature gas mass fromthe first stage rather than supplementary firing is used to heat thesecond half of the CO to ignition temperature. The distance between thefirst stage CO inlet and the second stage CO inlet depends on thecharacteristics of combustion in the first stage. It is desirable thatthe first stage combustion be 40-100 percent complete. The distance willbe from 1.5 /A to 0.5 /A where A is the area of a cross-section throughthe combustor at the CO inlet. The quantity of air added in the secondstage will range from 2.5-4.5 times the quantity of CO introduced intothe second stage.

The temperature of the combustion gases following combustion of the COwill depend on combustor condi- 4 tions, etc. Generally, the temperaturewill range from 16002200 F.

The CO furnace of our invention is unique in that it provides atransition section in which the flow of the combustion gases is turnedthrough of are from horizontal flow to vertical fiow. The transitionsection features a hanging wall 24 at the inlet and a sloped floor 33.The hanging wall or bafiie wall and the sloped floor cooperate to turnand to spread the rapidly moving gases to provide an even flow of hotgas over the surfaces of all the tubes in the convection section.

The first set of tubes 34 in the convection or heat transfer section aresteam tubes. By first we means to convey that these tubes are contactedfirst with the combustion gases. Water enters by line 15 and steam isremoved by line 16. Any suitable number of tube passes can be providedfor the steam making operation.

The hot but partially spent combustion gases are next used to heat apetroleum process stream and in a preferred embodiment the feed to acatalytic cracking unit. The second set of tubes shown generally byreference numeral 35 includes an oil inlet line 17, a set of tubes 36 inwhich the liquid progresses downwardly through the heat transfer sectioncountercurrent to combustion gas flow, a crossover line 18 which movesthe partially heated liquid to the bottom tube of a set of tubes 37 inwhich the flow of fluid is concurrent with the flow of the hotcombustion gases and an oil outlet line 19. We have found that thiscombination of countercurrent flow in the relatively cooler area andconcurrent flow in the relatively hotter area of the convection zoneinsures acceptable heat transfer rates to the oil in the critical tubes.This design permits heating of the catalytic cracking feed totemperatures in the range of 750850 F. without cracking of the lightercomponents of the feed in the convection section of the furnace.

The following example will serve to illustrate the invention.

A CO furnace was designed to recover 300MM B.t.u./ hr. of available heatby combusting the CO in regenerator gas from the regenerator of acatalytic cracking unit. The hot products of combustion are to be usedto provide 300MM B.t.u./hr. of catalytic cracker feed and to generatep.s.i.g. steam. The regenerator gas temperature is 450 F and the gasflow rate is 780,000 lb./hr. Employing a high heat release fuel, i.e.refinery fuel gas, having a higher heating value of 2l,00024,000 B.t.u./1b., it was found that a conventional single stage combustor wouldrequire 340MM B.t.u./hr. of supplementary firing to burn the CO comparedto 220MM B.t.u./hr. required for a two-stage combustor. Due to thehigher percentage of supplementary firing the conventional combustoroperates with a higher exit temperature than the dual stage combustor(1975 F. v. 1825 F.). However, this is a detriment rather than anadvantage because more expensive construction materials must be used inthe combustor and the convection section.

Thus, the multi-stage combustion of low heating value fuels is a meansfor significantly reducing the amount of high heating value fuelrequired for complete combustion of the low heating value fuel. Themulti-stage technique provides combustor flexibility and control aswell. By manipulating the dampers on the ducts which distribute thegases in each stage, gas flow to each stage can be regulated anddistributed to provide optimum combustion for various firing conditions.

The description of the invention given above in connection with thedrawings is for illustrative purposes only and is not to be considered alimitation on the process and apparatus of the invention. Obviousmodifications which would occur to those skilled in the art are intendedto be included in the scope of the specification and the claims.

What is claimed is:

1. A process for the combustion of low heating value gaseous fuelshaving a temperature of from about 400 to about 700 F. containing fromabout -15 volume percent CO and from about 80 to about 95 volume percentinert gases comprising sequentially burning a high heating value fuel toproduce hot combustion gases, mixing the hot combustion gases with aportion of the low heating value fuel and burning the CO in the lowheating value fuel in a first stage, mixing the hot combustion gasesfrom the first stage with the remainder of the low heating value fueland burning the CO in the loW heating value fuel in a second stagewhereby a reduced quantity of high heating value fuel is required.

2. The process according to claim 1 in which the portions of low heatingvalue fuel in each stage are approximately equal in volume.

3. A process for the combustion of a gas having a temperature of 400-700F. and containing 515 volume percent CO and 8095 volume percent inertgases consisting essentially of the steps of sequentially burning a fuelhaving a heating value of 21,000 to 24,000 B.t.u./lb. to providecombustion gases having a temperature of ll50l200 F., mixing the hotcombustion gases with approximately 50 volume percent of the stream COcontaining gas and burning the CO in said gas in a first stage, mixingthe first stage combustion gases with the other 50 volume percent of theCO containing gas and burning the CO in said gas in a second stagewhereby a combustion gas having a temperature in the range of from about1600 to about 2200" F. is produced.

4. A process for generating steam and for preheating hydrocarbon oil forcatalytic cracking in a catalytic cracking unit comprising a crackingunit and a cracking catalyst regenerator unit comprising the steps ofpassing regenerator off gas containing 5-15 volume percent CO from theregenerator to a CO furnace at a temperature of 400700 F., mixing from3070 percent of the off gas with the products of combustion of a highheat value fuel at a temperature of 1150-1200 F. in a first stage,burning the CO in the off gas, mixing the first stage combustion gaseswith from 30-70 percent of the off gas in a second stage whereby acombustion gas stream having a temperature of 16002200 F. is provided,contacting water and hydrocarbon oil with the combustion gas stream,converting the water to steam and heating the hydrocarbon oil to atemperature sufficient for use as catalytic cracking feed and passingthe oil to the said catalytic cracking unit.

5. A furnace for the combustion of low heat value fuels comprising wallsdefining a horizontal combustion zone, a vertical heat transfer zone anda transition zone joining the two zones, said combustion zone having aplurality of low heat value fuel supply means spaced horizontally apartalong the walls of the combustion zone defining a plurality ofhorizontally spaced low heat value fuel combustion stages and burnermeans positioned in the end of said combustion zone to supply hotcombustion gases from the combustion of high heat value fuel to thecombustion zone.

6. A furnace for preheating oil for catalytic cracking with COcontaining catalyst regenerator olf gas comprising in combination wallsdefining a horizontal combustion zone, a vertical heat transfer zone anda transition zone joining these zones, burner means for introducing hotcombustion products derived from the combustion of high heat value fuelinto one end of the combustion zone, at least tWo horizontally spacedapart gas ducts for introducing CO containing gas into the combustionzone in a direction normal to the flow of the combustion products fromthe burner whereby at least two CO combustion stages are provided, ahanging wall between the combustion zone and the transition zone, anupwardly inclined floor in the transition zone, a section ofhorizontally spaced steam tubes in the heat transfer zone, a set ofhorizontally spaced oil tubes disposed above the steam tubes in the heattransfer zone, inlet means for supplying relatively cold CO containinggas from a cracking catalyst regenerator to the said gas ducts, andegress means adapted to supply hot oil from said oil tubes to acatalytic cracker.

References Cited UNITED STATES PATENTS 2/1948 Van Dornick 208146 3/1961Downs 158-1

4. A PROCESS FOR GENERATING STEAM AND FOR PREHEATING HYDROCARBON OIL FORCATALYTHIC CRACKING IN A CATALYTIC CRACKING UNIT COMPRISING A CRACKINGUNIT AND A CRACKING CATALYST REGENERATOR UNIT COMPRISING THE STEPS OFPASSING REGENERATOR OFF GAS CONTAINING 5-15 VOLUME PERCENT CO FROM THEREGENERATOR TO A CO FURNACE AT A TEMPERATURE OF 400-700*F., MIXING FROM30-70 PERCENT OF THE OFF GAS WITH PRODUCTS OF COMBUSTION OF A HIGH HEATVALUE FUEL AT A TEMPERATURE OF 1150-1200*F. IN A FIRST STAGE, BURNINGTHE CO IN THE OFF GAS, MIXING THE FIRST STAGE COMBUSTION GASES WITH FROM30-70 PERCENT OF THE OFF GAS IN A SECOND STAGE WHEREBY A COMBUSTION GASSTREAM HAVING A TEMPERATURE OF 1600-2200*F. IS PROVIDED, CONTACTINGWATER AND HYDROCARBON OIL WITH THE COMBUSTION GAS